Exemplary embodiments of this invention will be described with respect to large format printers, although the invention can also be practiced on other types of printers.
Large scale plotters typically support roll-form print media, i.e., a supply of paper or transparent film on a roll. The media is loaded into the printer, and is advanced along a media path to a print area. A swath-type printer includes a carriage mounted for scanning movement along a swath axis, transverse to the media path at the print area. Hereafter, the media path is known as the X-axis, and the scanning or swath axis is the Y-axis. For color printing, the carriage holds a plurality of ink-jet printheads, each for printing a different color ink, typically black, cyan, magenta and yellow. The printer will include a media drive mechanism for moving the media along the media path, and a carriage drive mechanism for scanning the carriage along the scan axis. The printer controller issues print control signals to cause the printheads to eject droplets of ink in a controlled manner to form a desired image or plot on the medium.
Ink-jet printing is based on accurate ballistic delivery of small ink droplets to exact locations onto the paper or other media. Typically the droplet placement occurs onto a grid of different resolutions, most common grids being 300.times.300 dpi or 600.times.600 dpi, although other solutions are continuously being considered. One key factor for sharp and high quality images stems from the accuracy of the droplet placement.
There are several contributors to droplet placement inaccuracies. Some of these arise from the printer and some other from the printhead. They can occur along the scan axis or the media path directions. Some inaccuracies are systematic, while some others follow random patterns. Some of these errors can also affect the correction algorithm itself.
Several factors contribute to error in paper movements. The media roll is typically mounted in the printer on an axis or spindle. The spindle is prevented from turning at idle by a friction brake. This creates "back-tension" which helps the media auto-alignment. The media auto-alignment process includes X-axis movements, i.e. movements along the media advance direction, and rotations of the paper to prevent skew and mispositioning of the paper on the print zone. These movements create some undesirable paper slip on the print zone that negatively affect dot placement. These errors affect both printing and also dot placement calibration.
Some other movements have been detected when advancing the paper with back-tension. These movements are due to irregularities on the pinch-wheels as well as different pressures between pinch-wheels and roller and media tensions along the X-axis.
These factors can create several problems. When calibrating Y-axis directionality, i.e. where Y is the carriage scan axis direction, the back-tension creates large media slips. These movements include advance errors, X-axis displacements and rotations about a vertical (Z) axis.
When calibrating scan-axis directionality for the reference printhead, some advances are needed to print and measure dot placement along the printhead length. Some paper slip during these measurements could affect the calibration, causing the dots printed before the paper advance to be in a different X axis coordinate after the paper advance, and therefore measured at an incorrect distance to the next printed dots.